The present invention relates to an ionization method or ion source for ionizing a matter contained in a solution under atmospheric pressure or similar pressure and a mass spectrometry or mass spectrometer using the ionization method or ion source, and also relates to a liquid chromatograph/mass spectrometer, a capillary electrophoresis system/mass spectrometer and a plasma mass spectrometer.
As the related art, three techniques may be taken as examples as follows.
The first one of the examples of the related art is a method used in a plasma mass spectrometer, as disclosed in JP-A-2-248854 (U.S. Pat. No. 4,999,492). FIG. 16 is a reference view showing the method. In the method, ions generated by inductively coupled plasma are introduced into a high vacuum through a differential evacuation portion. In this occasion, in order to reduce noises due to high-speed neutral particles and photons mainly generated by plasma, ions extracted by an ion extraction lens 19 through an ion take-out aperture 7 of the differential evacuation portion are deflected by a deflector 20 and introduced into a mass analysis portion 13 through an ion take-in aperture 12 so that the high-speed neutral particles and photons going straight are cut partially.
The second example of the related art is the technique which is disclosed in JP-A-7-85834. FIG. 17 is a reference view showing the technique. The technique is adapted not only to a plasma mass spectrometer but also to a liquid chromatograph/mass spectrometer using a mass spectrometer as a detector of a liquid chromatograph to separate a mixture sample in solution, and a capillary electrophoresis system/mass spectrometer using a mass spectrometer as a detector of a capillary electrophoresis system to separate a mixture sample in solution. In this occasion, noises in the detector are mainly caused not by high-speed neutral particles and photons but by small droplets flowing into a high vacuum through a differential evacuation portion. In the case of a liquid chromatograph/mass spectrometer or a capillary electrophoresis system/mass spectrometer, there is employed a method in which electrically charged droplets are basically generated by spraying a solution and solvent molecules are vaporized from the electrically charged droplets to thereby generate ions of sample molecules. Accordingly, the electrically charged droplets thus generated are not always vaporized thoroughly, so that small droplets which are not vaporized inevitably remain. The not-vaporized small droplets flow into the high vacuum through the differential evacuation portion and reach the detector to cause big noises. In this technique, a double-cylindrical electrostatic lens is used as an electrostatic lens for deflecting and focusing ions. In this occasion, a large number of apertures are opened in an inner cylindrical electrode 10, so that ions are deflected and focused by using an electric field coming from the apertures of the inner cylindrical electrode 10 by the change of the voltage between the inner cylindrical electrode 10 and an outer cylindrical electrode 11 to thereby remove the small droplets, or the like, as the cause of noises.
The third example of the related art is the technique which is a method described in EP-A-0237249. FIG. 18 is a reference view showing the method. In the method, three quadrupole sets employing a high-frequency electric field are used. A first quadrupole set 26 has a function for mass-analyzing or focusing ions generated by an ion source 24 and focused by a lens 25. A second quadrupole set 27 is bent with a certain curvature. A detector 14 is disposed in the rear of a third quadrupole set 28 which has a function for mass-analyzing ions. Because the second quadrupole set 27 is bent with a certain curvature, ions having electric charges pass through the curved quadrupole set but neutral particles and droplets having no electric charges go straight. Accordingly, the neutral particles and droplets do not reach the detector 14 disposed in the rear of the third quadrupole set 28 for mass-analyzing ions, so that the noise level in the detector 14 is reduced correspondingly.
In the above first example, if the quantity of ion deflection is increased, the flowing of neutral particles, photons, etc., into the mass analysis portion can be prevented so that the noise level in the detector can be reduced correspondingly. If the quantity of ion deflection is increased, however, it becomes correspondingly difficult to focus ions again at the ion take-in aperture 12 of the mass analysis portion after deflection of ions. This is because the ion beam is widened at the ion take-in aperture 12 of the mass analysis portion or the angle of ions incident to the ion take-in aperture 12 of the mass analysis portion is increased. If the focus condition at the ion take-in aperture 12 of the mass analysis portion is poor, the ion transmission efficiency through the mass analysis portion becomes low so that the ion intensity of a sample to be measured, that is, the signal intensity is lowered. Accordingly, in the method, the signal intensity is reduced simultaneously with the reduction of noises even in the case where noises caused by high-speed neutral particles or photons are reduced by high ion deflection, so that it is finally impossible to improve greatly the signal-to-noise ratio as an index of detecting sensitivity.
Although the above description has shown the case where a quadrupole mass spectrometer is used as the mass spectrometer, this problem will become more serious when a special mass spectrometer such as an ion trap mass spectrometer, or the like, is used in the first example of the related art. In the case of a quadrupole mass spectrometer, the ion take-in aperture 12 of the mass analysis portion has a relatively large diameter of about 3 mm. Accordingly, even in the case where the focus condition at the ion take-in aperture 12 of the mass analysis portion is poor, that is, the ion beam is spread at the ion take-in aperture 12 of the mass analysis portion, the transmission efficiency of ions is not so greatly reduced. In the case of an ion trap mass spectrometer of the type in which ions are enclosed in a region surrounded by a pair of an end cap electrode and a ring electrode, however, the ion take-in aperture provided in the end cap electrode cannot be made so large because the disturbance of a high-frequency electric field in the inside cannot be made so large. Generally, the diameter of the ion take-in aperture in the case of an ion trap mass spectrometer is about 1.3 mm, which is smaller than that. in the case of a quadrupole mass spectrometer. Accordingly, in the case of an ion trap mass spectrometer, it has been confirmed that the lowering of the transmission efficiency of ions becomes remarkable if the ion beam is spread at the ion take-in aperture when ions are deflected in the manner as described above.
Also in the second example of the related art, the signal intensity is reduced simultaneously with the reduction of noises even in the case where ions are deflected greatly to reduce noises caused by droplets and neutral particles, the signal-to-noise ratio as an index of detecting sensitivity finally cannot be improved greatly.
In the third example of the related art, the apparatus becomes not only very complex but also very expensive. The quadrupole sets are required to be mechanically finished with accuracy of the order of microns, and the electrodes in the second quadrupole set are required to be bent with a certain curvature. Furthermore, a high-frequency electric source must be used in the quadrupole sets. Particularly in the case where electrodes in the second quadrupole set are bent with a large curvature in order to reduce noises greatly, there arises a serious problem in machining.